1. Technical Field
The present invention relates to a bag opening method and apparatus used to open bags in bag filling and packaging. More particularly, the present invention relates to a bag opening method and apparatus for use in bag filling and packaging in which a bag is gripped at its laterally opposite side edges with a pair of grippers (left and right grippers) and moved along a predetermined path to undergo predetermined packaging processes successively. Even more particularly, the present invention relates to a bag opening method and apparatus that are applicable to a plurality of different kinds of bags having different widths and that are capable of surely detecting whether or not each bag has been opened satisfactorily.
2. Background Art
Bag filling and packaging machines include those of the type in which a bag is gripped at its laterally opposite side edges with a pair of grippers (left and right grippers) and moved along a predetermined path to undergo predetermined packaging processes successively. In this type of packaging machine, each bag is supplied in a state where its mouth is closed, and the bag mouth is opened by a bag opening apparatus of the packaging machine. In the case of handling so-called self-supporting bags, the packaging machine uses a bag opening apparatus that opens the mouth of each bag and that unfolds and expands the bottom portion of the bag as well.
In a bag filling and packaging machine, if a bag fails to open satisfactorily, the subsequent operations such as the filling step cannot be accurately performed, resulting in a defective. In order to prevent such a problem, it is necessary to determine whether or not the bag has been opened satisfactorily prior to the operations such as the filling step. Japanese Patent Application Publication No. Hei 08-40420, for example, discloses a bag opening method which determines whether or not a bag has been opened satisfactorily.
According to the method disclosed in the above-described JP Publication, a pair of suction cups positioned at their standby positions the spacing between which is a specified spacing S are first advanced toward each other. When they reach their respective closest positions, the suction cups abut against the respective outer surfaces of the opposite side walls of a bag that is closed, and are stopped. Then a vacuum is applied to the suction cups through a vacuum line from a vacuum source to cause the suctions cups adhere to the bag. After the suction cups have been stopped at the closest positions for a predetermined time period, the suction cups are retracted away from each other by a first opening operation so that the spacing between the suction cups reaches a spacing S1 (intermediate position) shorter than the specified spacing S (standby position), and when the suction cups reach the respective intermediate positions a synchronizing signal is issued to measure the vacuum acting upon the suction cups at that time. After the suction cups have been stopped for a predetermined time period at the intermediate position or following the first opening operation, a second opening operation is performed to retract the suction cups away from each other to their respective standby positions where the spacing between the suction cups reaches the specified spacing S. Here the followings are defined;
t1: the time during which the first opening operation is performed, namely the time it takes for the suction cups to move from their respective closest positions to their respective intermediate positions,
t0: the time during which the suctions cups adhering to the bag stop at their closest positions, and
t: the time it takes for the vacuum in the vacuum pipeline to stabilize after start of application of vacuum to the suctions cups.
According to this method, the time t1 is set substantially the same as the time t. Thus, the sum of the time t1 and the time t0 is made longer than the time t to allow the vacuum in the vacuum pipeline to have surely stabilized when the synchronizing signal is issued. It should be noted that the vacuum is released to detach the suction cups from the bag immediately before the spacing between the suction cups reaches the specified spacing S, i.e. in synchronism with the time when the degree of opening of the bag reaches a maximum.
In the above-described Japanese Patent Application Publication No. Hei 08-40420, the bag opening method is disclosed as a technique to be applied to suction cups used to adhere to the side walls at portions near the bottom of what is called a self-supporting bag. This technique is capable of accurately determining whether or not each bag has been opened satisfactorily with regard to bags having a particular width by setting the spacings S and S1 according to the width of the bags and further setting the retracting speed of the suction cups according to the spacings S and S1.
Providing different packaging machines for different kinds of bags having different widths costs a great deal in terms of plant and equipment investment. It is therefore general practice to perform bag filling on a plurality of different kinds of bags having different widths using the same bag filling and packaging machine. In this case, generally, the spacing S1 (intermediate position) is set constant in conformity with the width of bags having the smallest width of all bags to be used, and the specified spacing S (standby position) is set constant in conformity with the width of bags having the largest width of all bags to be used. The timing of when the suction cups to adhere to a bag and when they release the suction cups from the bag, i.e. the vacuum application and release timings, are also typically set constant from the viewpoint of operation efficiency.
FIG. 7 illustrates operations of the prior art. In this case, the adhesion and detachment of the suction cups to and from the bag and the movement and stopping of the suction cups are as shown in the time chart of FIG. 4. In FIG. 4, the term “standby position” refers to where the suction cups are at respective positions at which the spacing therebetween is S, i.e. the specified spacing S as stated in Japanese Patent Application Publication No. Hei 08-40420 . The term “intermediate position” refers to where the suction cups are at respective positions at which the spacing therebetween is S1, which is less than the specified spacing S. The term “closest position” refers to where the suction cups are at respective positions at which the suction cups abut against the respective outer surfaces of the opposite side walls of a closed bag. The vacuum detection with the sensor is performed during the period of time that the suction cups are at the intermediate positions.
In the case of bags having a narrow width, the spacing S1 is set in conformity with bags having the smallest width, as stated above. Therefore, when the suction cups move from the first separating position (intermediate position) where the spacing is S1 toward the second separating position (standby position) where the spacing is S, the bag is forcibly separated from the suction cups adhering thereto (at point a in FIG. 7, for example). It has been found that a side wall of a bag will sometimes move toward the other side in reaction to this forced separation, resulting in the bag mouth not remaining open as much as desired (see FIG. 8). Accordingly, although the apparatus had judged that the bag had been opened satisfactorily on the basis of the vacuum detected just before the suction cups separated from the bag, the bag actually may have reclosed i.e. resulting in an operation which failed to open the bag satisfactorily or may have partially closed after detachment of the suction cups. In the case of bags having a wide width, the application of the vacuum to the suction cups is typically stopped shortly before the spacing reaches S (at point b in FIG. 7, for example). Thus, the bag opening operation is stopped at that time, and the suction cups separate from the bag and retract to their respective positions where the spacing therebetween is S (specified spacing). However, if the vacuum leaks before the cups reach the vacuum application stop position, e.g. due to deterioration of either suction cup or dislocation of the adhering position of the cup, then the suction cup also detaches from the bag undesirably, and one side wall of the bag moves toward the other side (see FIG. 9). In this case also, although the apparatus had judged that the bag had been opened satisfactorily, the bag actually has failed to open satisfactorily.
Japanese Patent Application Publication No. Hei 06-156437 discloses a technique wherein a pair of suction members 10 and 11 is initially positioned spaced from the bag side walls. The spacing is determined according to the width of bags to be used to dispose the suction members with a predetermined spacing therebetween in advance, and when a bag is placed between the suction members, the suction members are caused to suck the opposite side walls of the bag with a suction force generated by a suction pump, thereby opening the bag mouth. This bag opening method is totally different in basic principle from the bag opening method of Japanese Patent Application Publication No. Hei 08-40420, in which the suction cups are first caused to adhere to the respective outer surfaces of the opposite side walls of a closed bag, and subsequently the suction cups are retracted away from each other to open the bag. With the bag opening method of Japanese Patent Application Publication No. Hei 06-156437, the suction members are separate from the bag when the suction of the bag is started. Therefore, it is necessary to apply a very strong suction force in order to open the bag mouth with the suction members set separate from the bag. In addition, bags are usually made of a synthetic resin and hence the opposite side walls of the bag mouth are difficult to separate from each other due to the action of static electricity in addition to the fact that the bag mouth is merely physically closed. Accordingly, if the balance of vacuums acting on the two suction members, respectively, is destroyed, even slightly, the bag is undesirably drawn by suction to either of the suction members without being opened. Japanese Patent Application Publication No. Hei 06-156437 does not disclose any means for detecting such a failure in opening bags.